Hot-rolled hot-dip galvanized steel sheet having excellent surface appearance and method for manufacturing the same

Abstract

According to one aspect of the invention, a hot-rolled hot-dip galvanized steel sheet having an excellent surface appearance may include a base steel sheet and a galvanized layer, the galvanized layer including a continuous Al-enriched layer formed from the surface of the galvanized layer to within 100 nm.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201880084538.1, filed on December 6, 2018, entitled "Hot-rolled hot-dip galvanized steel sheet with excellent surface appearance and manufacturing method thereof", and claims priority to Chinese Patent Application No. 10-2017-0180298 filed with the Korean Intellectual Property Office (KIPO) on December 26, 2017, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to a hot-rolled hot-dip galvanized steel sheet and its manufacturing method. More specifically, this invention relates to a hot-rolled hot-dip galvanized steel sheet with an excellent surface appearance by effectively suppressing surface defects in the coating and its manufacturing method. Background Technology

[0003] Hot-dip galvanized steel sheet refers to steel sheet with a galvanized layer formed on the surface of a base steel sheet through hot-dip galvanizing. Hot-rolled hot-dip galvanized steel sheet refers to steel sheet with a galvanized layer formed on the surface of a hot-rolled steel sheet through hot-dip galvanizing. Typically, hot-rolled hot-dip galvanized steel sheet can be manufactured through a series of processes such as scale breaking, pickling, heat treatment, immersion in a galvanizing bath, and air wiping.

[0004] Pickling is a process implemented to remove oxide scale formed during hot rolling. It primarily utilizes acidic solutions to remove oxide scale from the surface of hot-rolled steel sheets through chemical pickling. However, this chemical pickling process is environmentally harmful due to the use of strong acidic solutions such as sulfuric acid or hydrochloric acid, and it severely corrodes the base material as the reaction time increases. Therefore, there is a need for an oxide scale removal technology that can effectively remove oxide scale from the surface of hot-rolled steel sheets while minimizing environmental pollution.

[0005] Furthermore, the air wiping process is essential for adjusting the coating amount on the steel sheet surface by spraying high-pressure fluid after immersion in the plating bath. However, spraying high-pressure fluid onto the coating before it has fully solidified can lead to surface defects such as flow marks. A relatively low-flow-rate oxide film forms on the surface of the coating, while hot-dip galvanizing has relatively high flow-rate inside, creating a difference in flowability along the thickness direction of the coating. When the high-pressure fluid sprayed from the air knife reaches the surface of the coating, it causes cracks to form in the oxide film, exposing the molten zinc inside the coating. After passing through the air knife, the coating solidifies rapidly, resulting in continuous flow mark defects with valley and peak shapes on the surface of the coating.

[0006] In the prior art, a technique has been proposed to prevent flow mark defects by introducing a sealing box to create a non-oxidizing atmosphere, thereby preventing the formation of the oxide film itself. However, while introducing a sealing box can suppress the formation of an oxide film on the surface of the coating to some extent, the equipment structure becomes overly complex. Furthermore, when a gap exists between the plating bath and the sealing box to facilitate the removal of evaporated zinc, excessive scum is generated on the surface of the plating bath due to the reaction between external air and the plating bath, which actually reduces the surface quality of the coated steel sheet.

[0007] Patent document 1 proposes a technique that mechanically removes flow mark defects by leveling and rolling after coating formation, rather than suppressing the formation of flow mark defects themselves. However, to remove flow marks, the steel sheet needs to be pressurized with the highest possible reduction force, thus posing a high risk of damaging the normal coating and causing coating peeling. Therefore, there is an urgent need to introduce a technique that can effectively suppress flow mark defects on the coating surface to prevent a reduction in the surface quality of the coated steel sheet.

[0008] (Patent Document 1) Korean Patent Publication No. 10-2001-0060423 (published on July 7, 2001) Summary of the Invention

[0009] (a) Technical problems to be solved According to one aspect of the present invention, a hot-rolled hot-dip galvanized steel sheet with excellent surface appearance and a method thereof can be provided.

[0010] The technical problems to be solved by this invention are not limited to those described above. From the entirety of this specification, those skilled in the art will readily understand additional technical problems addressed by this invention.

[0011] (II) Technical Solution According to one aspect of the invention, a hot-rolled hot-dip galvanized steel sheet may include a base steel sheet and a galvanized layer, said galvanized layer may include a continuous Al-enriched layer formed from the surface of said galvanized layer to a depth of 100 nm.

[0012] The Al-enriched layer can be an alumina (Al2O3) layer.

[0013] The thickness of the Al enrichment layer can be less than 50 nm (excluding 0 nm).

[0014] Based on the centerline average roughness (Ra), the surface roughness of the base steel plate surface forming the interface with the galvanized layer can be 0.7μm to 2.5μm.

[0015] The zinc plating layer may contain 0.2% to 0.6% Al and the balance Zn and other unavoidable impurities, by weight%.

[0016] According to one aspect of the invention, a hot-rolled hot-dip galvanized steel sheet can be manufactured by the following process: stretching a base steel sheet at a first elongation to generate cracks in the oxide scale on the surface of the base steel sheet; applying physical impact to the surface of the base steel sheet to mechanically remove the oxide scale from the surface of the base steel sheet; reacting the surface of the base steel sheet with an acidic solution to chemically pickle the surface of the base steel sheet; leveling the base steel sheet at a second elongation to flatten the surface of the base steel sheet; and immersing the base steel sheet in a hot-dip galvanizing bath containing 0.2% to 0.4% Al by weight and the balance Zn and other unavoidable impurities to form a galvanized layer.

[0017] The first elongation can be 0.2~1.5%.

[0018] The surface of the base steel plate can be shot-blasted to mechanically remove the oxide scale from the surface of the base steel plate.

[0019] The shot peening process can project projectiles with an average diameter of 0.18 mm to 0.6 mm at an average projection rate of 800 kg / min to 1800 kg / min and an average projection velocity of 65 m / s to 90 m / s.

[0020] The base steel plate can be immersed in a hydrochloric acid solution with a temperature range of 70℃ to 85℃ and a concentration of 5% to 20% for 15 to 35 seconds to chemically pickle the surface of the base steel plate.

[0021] The base steel plate can be leveled and rolled using a smooth roller. The surface roughness of the smooth roller can be 0.1μm to 0.8μm, with the centerline average roughness (Ra) as the reference.

[0022] Based on the centerline average roughness (Ra), the surface roughness of the leveled and rolled base steel plate can be 0.7μm~2.5μm.

[0023] The second elongation can be 0.5% to 2.5%.

[0024] The sum of the first elongation and the second elongation can be 0.7% to 4.0%.

[0025] The second elongation can be greater than the first elongation.

[0026] The immersion temperature of the hot-dip galvanizing bath for the base steel plate can be 450℃~500℃.

[0027] (III) Beneficial Effects According to one aspect of the present invention, a hot-rolled hot-dip galvanized steel sheet and a method thereof can be provided to effectively suppress the generation of flow mark defects on the surface of the galvanized layer, thereby achieving an aesthetically pleasing surface appearance.

[0028] According to one aspect of the present invention, a method for manufacturing hot-rolled hot-dip galvanized steel sheet can be provided, wherein the base steel sheet is subjected to mechanical removal of oxide scale and chemical pickling in sequence, thereby minimizing the use of chemical solutions for chemical pickling while effectively removing residual oxide scale from the surface of the base steel sheet. Attached Figure Description

[0029] Figures 1 to 3 The results are obtained by analyzing the surface layer of a hot-rolled hot-dip galvanized steel sheet according to one aspect of the present invention using focused ion beam transmission electron microscopy (FIB-TEM).

[0030] Figures 4 to 6 The results are from FIB-TEM analysis of the surface layer of hot-rolled hot-dip galvanized steel sheets with discontinuous alumina layers. Detailed Implementation

[0031] This invention relates to a hot-rolled hot-dip galvanized steel sheet with excellent surface appearance and a method for manufacturing the same. Preferred embodiments of the invention will be described below. The embodiments of the invention can be modified in various ways, and the scope of the invention should not be construed as limited to the embodiments described below. These embodiments are provided to describe the invention in more detail to those skilled in the art.

[0032] According to one aspect of the present invention, a hot-rolled hot-dip galvanized steel sheet may comprise a base steel sheet and a galvanized layer formed on the surface of the base steel sheet. The base steel sheet of the present invention may be a hot-rolled steel sheet, but is not limited thereto. It can be interpreted as including all steel sheets capable of being galvanized.

[0033] According to one aspect of the present invention, a hot-rolled hot-dip galvanized steel sheet may include an Al-enriched layer continuously distributed within the galvanized layer to a depth of 100 nm from the surface of the galvanized layer. That is, the Al-enriched layer of the present invention may be continuously distributed from the surface of the galvanized steel sheet to a predetermined depth in a direction parallel to the surface of the galvanized steel sheet. Furthermore, the thickness of the Al-enriched layer of the present invention may be 50 nm or less (excluding 0 nm). In other words, the Al-enriched layer of the present invention is formed from the surface of the galvanized steel sheet to a predetermined depth and is formed to a predetermined thickness, and may be continuously distributed in a direction parallel to the surface of the galvanized steel sheet.

[0034] The Al-enriched layer of the present invention can be an alumina (Al₂O₃) layer. Although the Al content is relatively small compared to the Zn content in the zinc coating of the present invention, an alumina (Al₂O₃) enriched layer can be continuously formed in the surface portion of the zinc coating. This is because Al is an element with a high oxygen affinity compared to Zn, and during coating formation, Al within the coating moves to the surface portion of the coating and combines with oxygen to form oxides. That is, in the surface portion of the zinc coating of the present invention, an alumina layer can be formed before zinc oxide is formed, and by forming a continuous alumina layer, zinc oxidation on the surface of the alumina layer can be suppressed.

[0035] According to one aspect of the present invention, the base steel plate of the hot-dip galvanized steel sheet has a surface roughness of 0.7 μm to 2.5 μm, based on the centerline average roughness (Ra). That is, the base steel plate of the present invention has a flat surface with a surface roughness of 0.7 μm to 2.5 μm based on the centerline average roughness (Ra), thus minimizing the deviation in the degree of oxidation on the surface of the base steel plate, thereby enabling the formation of an aluminum oxide layer distributed in a continuous morphology during coating.

[0036] Figures 1 to 3 The results are obtained by using FIB-TEM analysis of the surface layer of a hot-rolled hot-dip galvanized steel sheet according to one aspect of the present invention. Figures 4 to 6 The results are from FIB-TEM analysis of the surface layer of hot-rolled hot-dip galvanized steel sheets with discontinuous alumina layers.

[0037] Figures 1 to 3 The results are shown by analyzing the same cross-section of a hot-rolled hot-dip galvanized steel sheet according to one aspect of the present invention using FIB-TEM, and the distributions of Zn, Al, and O are illustrated respectively. Figure 2 and Figure 3 As shown, it can be confirmed that Al and O are continuously distributed on the surface side of the zinc plating layer, and an alumina-type enriched layer is continuously formed on the surface side of the zinc plating layer. Furthermore, as... Figure 1 As shown, it can be confirmed that Zn is located in the lower part of the alumina, therefore almost no Zn is present on the surface of the alumina layer. That is, it can be confirmed that a zinc-deficient layer is formed on the surface of the alumina layer, therefore almost no zinc oxide is present on the surface side of the zinc plating layer.

[0038] On the other hand, such as Figures 4 to 6As shown, it can be confirmed that in the case of a zinc plating layer with a discontinuous alumina layer, Al and O are discontinuously distributed on the surface side of the plating layer, and the alumina layer is formed discontinuously. That is, it can be confirmed that the reaction of Zn and O occurs at the discontinuous locations of the alumina layer, thus, uneven zinc oxide is exposed on the surface of the plating layer. Therefore, when an alumina layer is discontinuously present on the surface side of the plating layer, during the air wiping operation after immersion in the plating bath, the zinc oxide exposed to the outside of the plating layer cracks, and molten zinc inside the plating layer flows out to the outside of the plating layer, thereby causing flow mark defects.

[0039] Furthermore, by weight percent, the zinc coating of the present invention may contain 0.2% to 0.6% Al and the balance Zn and other unavoidable impurities. The composition of the zinc coating of the present invention is affected by the composition of the hot-dip galvanizing bath described later; therefore, the description of the composition of the zinc coating of the present invention will be replaced by the description of the composition of the hot-dip galvanizing bath described later. However, in typical hot-dip galvanizing, the Al content in the coating is slightly higher than the Al content in the galvanizing bath; therefore, the Al content in the zinc coating of the present invention may be slightly higher than the Al content in the hot-dip galvanizing bath. That is, the upper limit of 0.6% Al content in the zinc coating of the present invention is a range taking the above-mentioned situation into account.

[0040] According to one aspect of the present invention, the hot-rolled hot-dip galvanized steel sheet has a continuous aluminum oxide (Al2O3) layer on the surface of the galvanized layer, thereby effectively preventing the formation of uneven zinc oxide on the surface of the galvanized layer, and thus effectively suppressing flow mark defects formed on the surface of the hot-dip galvanized steel sheet.

[0041] The manufacturing method of the present invention will now be described in more detail.

[0042] According to one aspect of the present invention, a hot-rolled hot-dip galvanized steel sheet can be manufactured by the following process: stretching a base steel sheet at a first elongation to generate cracks in the oxide scale on the surface of the base steel sheet; applying physical impact to the surface of the base steel sheet to mechanically remove the oxide scale from the surface of the base steel sheet; reacting the surface of the base steel sheet with an acidic solution to chemically pickle the surface of the base steel sheet; leveling the base steel sheet at a second elongation to flatten the surface of the base steel sheet; and immersing the base steel sheet in a hot-dip galvanizing bath containing 0.2% to 0.4% Al by weight and the balance Zn and other unavoidable impurities to form a galvanized layer.

[0043] According to one aspect of the present invention, a method for manufacturing hot-rolled hot-dip galvanized steel sheet involves stretching a base steel sheet at a first elongation rate in a scale breaker to remove scale from the surface of the base steel sheet. Then, residual scale on the surface of the base steel sheet is removed by shot blasting. Finally, the shot-blasted base steel sheet is immersed in an acidic solution to pickle the remaining scale on its surface. Therefore, hot-rolled scale generated during the manufacturing process of hot-rolled steel sheet can be effectively removed, thereby effectively ensuring the cleanliness of the base steel sheet surface.

[0044] According to one aspect of the present invention, a method for manufacturing hot-rolled hot-dip galvanized steel sheet involves flattening a base steel sheet by removing oxide scale to impart roughness, thereby flattening the surface of the steel sheet. The base steel sheet can then be immersed in a hot-dip galvanizing bath to form a galvanized layer. That is, by ensuring that the surface flatness of the base steel sheet is above a predetermined level through flattening rolling, the oxidation degree deviation of the base steel sheet surface can be effectively reduced, thereby enabling the formation of a continuously distributed Al-enriched layer.

[0045] The following will describe in more detail each process condition constituting the manufacturing method of the present invention.

[0046] Oxide scale removal (scale breaking) Oxide scale removal can be performed as a preliminary process for removing hot-rolled oxides formed on the surface of a base steel plate. During oxide scale removal, cracks can be generated in the hot-rolled oxides formed on the surface of the base steel plate due to the base steel plate being stretched at a first elongation. Therefore, by generating cracks in the hot-rolled oxides on the surface of the base steel plate through oxide scale removal, the efficiency of subsequent mechanical and chemical oxide scale removal can be effectively improved.

[0047] To fully achieve the desired effect of removing residual oxide scale during mechanical removal and chemical pickling, the elongation rate during oxide scale removal should be above a predetermined level. Even with mechanical removal and chemical pickling, if a predetermined amount of residual oxide scale remains, uncoated or peeling coating may occur. Therefore, the first elongation rate of this invention can be 0.2% or higher. On the other hand, when the first elongation rate exceeds the predetermined level, the base steel plate material hardens, and even leveling rolling cannot adequately achieve a flattening effect. Therefore, the first elongation rate of this invention can be limited to 1.5% or less.

[0048] Mechanical removal of oxide scale Shot peening can be applied to base steel plates that have already had their oxide scale removed. Shot peening is performed by projecting tiny shot balls onto the surface of the base steel plate. The impact of the projectiles accelerates the growth of cracks formed in the oxide scale on the surface of the base steel plate, thereby allowing the remaining oxide scale to detach from the surface of the base steel plate.

[0049] The diameter of the shot used in the shot peening process of this invention can be 0.18 mm to 0.6 mm. This is because when the diameter of the shot is less than 0.18 mm, the impact applied to the base steel plate is insufficient, reducing the efficiency of oxide scale removal. When the diameter of the shot exceeds 0.60 mm, it not only exceeds the impact required to remove oxide scale, but also may exacerbate the local unevenness of the impacted part of the steel plate.

[0050] In the shot peening process of this invention, the average shot delivery rate can be from 800 kg / min to 1800 kg / min. When the average shot delivery rate is too low, the probability of collision with the steel plate decreases, and the effect of removing residual oxide scale cannot be expected. Therefore, the average shot delivery rate can be 800 kg / min or higher. On the other hand, when the average shot delivery rate is too high, the cost may be excessive compared to the increased efficiency of oxide scale removal. Therefore, the average shot delivery rate can be 1800 kg / min or lower.

[0051] In the shot peening process of this invention, the average projectile velocity can be 65 m / s to 90 m / s. When the average projectile velocity is below a predetermined level, the impact transmitted to the base steel plate does not reach the predetermined level due to the reduced kinetic energy of individual projectiles; therefore, the average projectile velocity can be 65 m / s or higher. However, when the average projectile velocity is too high, the impact transmitted to the base steel plate may exceed the necessary amount, potentially exacerbating surface unevenness; therefore, the average projectile velocity can be 90 m / s or lower.

[0052] Chemical pickling Chemical pickling can be performed by reacting the surface of a pre-shot-peened steel plate with an acidic solution. Pickling efficiency is mainly affected by factors such as the concentration of the pickling solution, temperature, and reaction time; therefore, the efficiency of chemical pickling can be optimally managed by appropriately controlling these factors. Hydrochloric acid or sulfuric acid can typically be used as the pickling solution; however, compared to sulfuric acid, hydrochloric acid has stronger corrosiveness, excellent surface oxide scale removal ability, and the advantage of less hydrogen embrittlement. Therefore, hydrochloric acid solution can be used in the chemical pickling process of this invention.

[0053] Considering the pickling efficiency, the hydrochloric acid solution used for chemical cleaning in this invention can contain hydrochloric acid with a concentration of 5% or more. On the other hand, when the hydrochloric acid concentration is too high, the concentration of ferric chloride (FeCl2) reaches a supersaturated state and the reaction stops. The pickling efficiency near the precipitation point becomes constant or even decreases. Therefore, the hydrochloric acid solution used for chemical pickling in this invention can contain hydrochloric acid with a concentration of 20% or less.

[0054] To ensure pickling capability, the chemical pickling of the present invention can be carried out in a temperature range of 70°C or higher. On the other hand, when the temperature of chemical pickling is too high, the improvement in pickling capability is insufficient, but excessive pickling may lead to excessive corrosion of the base steel plate, and the evaporation of the acid solution increases rapidly, which is not economically desirable. Therefore, the chemical pickling of the present invention can be carried out at a temperature of 85°C or lower.

[0055] The chemical pickling of the present invention can be carried out for more than 15 seconds to provide sufficient time to remove the oxide scale remaining on the surface of the base steel plate. However, when the chemical pickling time is too long, excessive pickling may lead to excessive corrosion of the base steel plate, and the removal of chloride ions in the subsequent process takes a relatively long time, which is not preferred in terms of efficiency. Therefore, the chemical pickling of the present invention can be carried out for less than 35 seconds.

[0056] According to one aspect of the present invention, the method for manufacturing hot-rolled hot-dip galvanized steel sheet combines mechanical removal of oxide scale by shot peening with chemical pickling by hydrochloric acid solution. Therefore, compared with chemical pickling alone, oxide scale can be removed in a shorter time, and the amount of acidic solution used can be effectively reduced. Furthermore, according to one aspect of the present invention, the method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance combines mechanical removal of oxide scale by shot peening with chemical pickling by hydrochloric acid solution, thus effectively removing oxide scale remaining on the surface of the base steel sheet and effectively ensuring the cleanliness of the base steel sheet surface.

[0057] leveling rolling After mechanical removal of oxide scale and chemical pickling, leveling rolling can be performed by applying pressure to the surface of the base steel plate using a bright roll. Based on the average surface roughness (Ra) of the bright roll's centerline, an average roughness of 0.1 μm to 0.8 μm can be achieved, and the base steel plate can be stretched at a second elongation rate by applying pressure to the bright roll.

[0058] The lower limit of the second elongation rate used to ensure the flatness of the base steel plate surface can be above 0.4%. However, when the second elongation rate is too large, the surface roughness flattening effect of the base steel plate becomes saturated, and there are problems with the shape deformation and material hardening of the base steel plate due to excessive stretching. Therefore, the upper limit of the second elongation rate can be limited to 2.5%.

[0059] In the aforementioned oxide scale removal process, the base steel plate is stretched at a first elongation rate to induce cracks in the oxide scale on the surface of the base steel plate. In the leveling rolling process, the base steel plate is stretched at a second elongation rate to ensure surface flatness. Therefore, to achieve an effective surface roughness leveling effect, a second elongation rate greater than the first elongation rate used in oxide scale removal can be used to stretch the base steel plate during leveling rolling. This is because when the first elongation rate is greater than the second elongation rate, the material hardening generated during oxide scale removal makes it difficult to ensure sufficient surface roughness through leveling rolling.

[0060] In addition, to achieve surface cleanliness and roughness flattening of the base steel plate, the sum of the first elongation and the second elongation can be 0.7% or more. However, when the sum of the first elongation and the second elongation exceeds a predetermined level, it causes excessive rolling load in the leveling rolling mill, shortens the roll life, and may cause material deformation due to excessive reduction. Therefore, the sum of the first elongation and the second elongation can be 4% or less.

[0061] Based on the centerline average roughness (Ra), the surface roughness of the base steel plate after leveling and rolling can be at the level of 0.7 μm to 2.5 μm. During the initial immersion stage of the hot-dip galvanizing bath, highly reactive Al first reacts with Fe in the base steel plate to form an Fe-Al alloy phase, thereby inhibiting the growth of hard Fe-Zn intermetallic compounds. Therefore, when the effective reaction surface area of ​​the base steel plate increases, the growth of Fe-Zn intermetallic compounds is suppressed, which helps improve the mechanical and physical properties of the coating, thus effectively preventing coating peeling. Therefore, to achieve this effect, leveling and rolling can be performed to ensure that the surface roughness of the steel plate meets or exceeds 0.7 μm based on the centerline average roughness (Ra). On the other hand, when the roughness deviation of the base steel plate is too large, Al oxidation concentrates in areas with large roughness deviations, thereby locally forming an alumina layer to create a discontinuous alumina layer. Therefore, in order to ensure a continuous distribution of the alumina layer on the surface of the coating, a flat rolling process can be performed so that the surface roughness of the steel plate meets a requirement of less than 2.5 μm based on the centerline average roughness (Ra).

[0062] Hot-dip galvanizing bath immersion A pre-rolled and leveled base steel plate can be immersed in a zinc-based plating bath to form a galvanized layer. The hot-dip galvanizing bath of the present invention may contain 0.2% to 0.4% Al by weight, with the balance being Zn and other unavoidable impurities. Al is an element that imparts fluidity to the plating bath and helps improve the adhesion between the galvanized layer and the base steel plate; therefore, the hot-dip galvanizing bath of the present invention may contain more than 0.2% Al. However, when the added Al content is too high, the effect of improving the fluidity of the plating bath reaches saturation, and the formation of scum will increase due to the promotion of Fe corrosion; therefore, the Al content of the hot-dip galvanizing bath of the present invention may be less than 0.4%. Preferably, the Al content of the hot-dip galvanizing bath may be 0.2% to 0.24%.

[0063] The base steel plate of the present invention can be immersed in a hot-dip galvanizing bath at an immersion temperature of 450°C to 500°C. When the temperature of the base steel plate is lower than the temperature of the hot-dip galvanizing bath, the possibility of flow mark defects increases due to the reduced fluidity of the hot-dip galvanizing bath. Therefore, the base steel plate is preferably heated to a temperature higher than that of the galvanizing bath and held before being immersed in the galvanizing bath. Thus, the base steel plate of the present invention can be immersed in a hot-dip galvanizing bath at an immersion temperature of 450°C or higher. Furthermore, when the immersion temperature of the base steel plate is higher than that of the hot-dip galvanizing bath, Fe dissolution is accelerated, increasing the formation of dross and potentially leading to surface defects, such as dross imprinting on the coating surface. Therefore, the base steel plate of the present invention can be immersed in a hot-dip galvanizing bath at an immersion temperature of 500°C or lower, thereby forming a galvanized layer on the base steel plate.

[0064] Therefore, the hot-rolled hot-dip galvanized steel sheet of the present invention manufactured by the above manufacturing method can effectively suppress flow mark defects on the surface of the galvanized layer, thus achieving an aesthetically pleasing surface appearance.

[0065] In addition, the above manufacturing method sequentially performs mechanical removal of oxide scale and chemical pickling on the base steel plate, thus minimizing the use of chemical solutions for chemical pickling while effectively removing residual oxide scale from the surface of the base steel plate.

[0066] The present invention will now be described in more detail through embodiments. These embodiments are intended to describe preferred embodiments of the invention in more detail; however, it should be noted that the scope of the invention is not necessarily limited to these embodiments.

[0067] JS-SPHC (sample 1, tensile strength 350 MPa) with a thickness of 3.2 mm and JS-SAPH400 (sample 2, tensile strength 400 MPa) with a thickness of 2.9 mm were selected as test samples. Although the present invention is not necessarily applicable to thick hot-rolled steel plates, the selection of 3 mm thick hot-rolled steel plates provides a harsh environment for improving flow mark defects. The oxide scale on the surface of the test samples was removed using the first elongation rate described in Table 1 below, mechanical removal of oxide scale, and chemical pickling. Each test sample was then leveled and rolled using a smooth roller with a roughness (Ra) of 0.2 μm, and stretched at the second elongation rate. After leveling and rolling, the rolling oil was degreased and dried, and galvanizing was applied to the surface of the test samples. The plating was performed using the Multi Functional Process Simulator of Iwatani Corp., and hot-dip galvanized samples were manufactured by setting the heat treatment and plating conditions as shown in Table 2 below. When manufacturing hot-dip galvanized samples, the temperature conditions for each range (PHS, DFF, HRS, GJS, TDS) are the same, but only the immersion temperature of the base steel plate is set differently.

[0068] [Table 1] [Table 2] After plating, air wiping was performed under the same conditions. Table 3 below shows the results of observing the surface and coating of each hot-dip galvanized sample. The surface quality, distribution range, continuity, and maximum thickness of the Al-enriched layer were measured for each sample. Surface quality was evaluated by visual inspection of each sample; specifically, ○ indicates excellent (no flow marks or no plating), and × indicates insufficient (flow marks or no plating). After processing with FIB (Ion Beam Accelerator), the distribution range, continuity, and maximum thickness of the Al-enriched layer were analyzed by TEM (Transmission Electron Microscopy). Additionally, the residual oxide scale of each base steel plate was evaluated before plating each sample, and the results are also shown in Table 3. For the evaluation of residual oxide scale on the base steel plate, the fraction of the oxide scale area on the image was calculated by an image analyzer after confirming the 200x magnification scanning electron microscope (SEM) image in back-scattering mode.

[0069] [Table 3] In Examples 1 to 15, which meet the conditions of the present invention, it can be confirmed that a continuous Al-enriched layer is formed from the surface of the zinc plating layer to a depth of 100 nm or less, and the maximum thickness of the Al-enriched layer does not exceed 50 nm. Furthermore, in Examples 1 to 15, it can be confirmed that the residual oxide scale on the base steel plate before plating is less than 3%, ensuring excellent surface cleanliness and thus forming a continuous Al-enriched layer. Therefore, in Examples 1 to 15, it is confirmed that no flow marks or unplated surface defects occur, resulting in an excellent surface appearance.

[0070] On the other hand, in Comparative Examples 1 to 15, which do not meet the conditions of the present invention, it can be confirmed that the Al enriched layer is formed discontinuously, and the maximum thickness of the Al enriched layer exceeds 50 nm. That is, due to the formation of a non-uniform and discontinuous Al enriched layer, zinc oxide is formed unevenly on the surface of the coating, resulting in flow mark defects or uncoated surface defects.

[0071] Although the invention has been described in detail above by way of embodiments, other embodiments are also possible. Therefore, the technical concept and scope of the appended claims are not limited to the embodiments described above.

Claims

1. A hot-rolled hot-dip galvanized steel sheet with excellent surface appearance, comprising: Foundation steel plate; as well as Zinc plating layer The zinc plating layer comprises a continuous Al-enriched layer formed from the surface of the zinc plating layer to a depth of up to 100 nm.

2. The hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 1, wherein, The Al-enriched layer is an aluminum oxide (Al2O3) layer.

3. The hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 1, wherein, The thickness of the Al enriched layer is less than 50 nm, excluding 0 nm.

4. The hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 1, wherein, Based on the centerline average roughness (Ra), the surface roughness of the base steel plate surface forming the interface with the galvanized layer is 0.7μm~2.5μm.

5. The hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 1, wherein, The zinc coating contains 0.2% to 0.6% Al by weight and the balance Zn and other unavoidable impurities.

6. A method for manufacturing a hot-rolled hot-dip galvanized steel sheet with excellent surface appearance, comprising: The base steel plate is stretched at a first elongation rate to create cracks in the oxide scale on the surface of the base steel plate; The surface of the base steel plate is shot blasted to mechanically remove the oxide scale from the surface of the base steel plate; The base steel plate is immersed in a hydrochloric acid solution with a temperature range of 70°C to 85°C and a concentration of 5% to 20% for 15 to 35 seconds to chemically pickle the surface of the base steel plate. The base steel plate is flattened by a smoothing roller with a second elongation rate. The surface roughness of the base steel plate is 0.1 μm to 0.8 μm, with the centerline average roughness Ra as a reference. as well as The base steel plate is immersed in a hot-dip galvanizing bath consisting of 0.2% to 0.4% Al by weight, with the balance being Zn and other unavoidable impurities, to form a zinc coating. Wherein, the second elongation is greater than the first elongation.

7. The method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 6, wherein, The first elongation is 0.2~1.5%.

8. The method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 6, wherein, The shot peening process involves projecting projectiles with an average diameter of 0.18 mm to 0.6 mm at an average projection rate of 800 kg / min to 1800 kg / min and an average projection velocity of 65 m / s to 90 m / s.

9. The method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 6, wherein, Based on the average roughness Ra of the centerline, the surface roughness of the leveled and rolled base steel plate is 0.7μm~2.5μm.

10. The method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 6, wherein, The second elongation is 0.5% to 2.5%.

11. The method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 6, wherein, The sum of the first elongation and the second elongation is 0.7% to 4.0%.

12. The method for manufacturing hot-rolled hot-dip galvanized steel sheet with excellent surface appearance according to claim 6, wherein, The immersion temperature of the hot-dip galvanizing bath for the base steel plate is 450℃~500℃.

Images